R33 Gts-T Vacuum At Idle

[insert]

hi all,

recently bought the car. it has a drift iridium (digital) boost gauge. ad idle its reading -18psi. is that a normal/correct reading, or is the gauge malfunctioning. for some reason i think it should be reading less.

the car is pretty much bone stock mechanically save for a CAI, exhaust, intercooler, boost controller (running stock boost level).

forgive me for my ignorance, its my first r33.

thanks

[The Mafia]

Mine was -20psi at idle.

[insert]

alright, guess i have nothing to worry about

[tridentt150v]

If you ever get an EVC with a digital display, you can be doing 100km/h down the road and your boost is reading negative.

[zebra]

It wont be -18psi...it will be 18mmHg

[89CAL]

Yep Psi is a measure of positive pressure

Also I've found playing with the tune at the idle points i can get my values to change alot so i wouldn't worry to much about it

[GTSBoy]

Yep Psi is a measure of positive pressure

And negative pressure. Pressure units are quite universal. It's just that manifold vacuum is traditionally read in some of the more bullshit units like inches or mm of mercury. It shits me to tears that whilst we have a perfectly good set of self consistent SI units for pressure (Pa, dPa, hPa, kPa, MPa) that people continue to talk in rubbishy units based on the column height of some randomly selected fluid like mercury or water. It's time to come kicking and screaming into the 20th century is what I say.

[zebra]

Isnt it because Mercury isnt affected by air density as much?

Therefore measuring head flow in mmHg or inHg is more consistant, rather than saying x port flows y cfm at 22.14646363 deg C and 1009 QNH.

I think...lol

[zebra]

Wait...no....I'm way to tired to think about this stuff

[89CAL]

I've seen mercury and water used, there is a reason i just dontt remember

A negative psi value does really exist. you can't have below 0psi in a vessel. 0 pounds/square inch

Remembering that 0psi on your boost gauge is still 1atmosphere

[GTSBoy]

A negative psi value does really exist. you can't have below 0psi in a vessel. 0 pounds/square inch

Remembering that 0psi on your boost gauge is still 1atmosphere

Yeah you can. Absolutely no reason why you cannot express it in psi. The positive pressure is outside the vessel.

psi is a measure of force (pounds force) per unit area (square inch). So is any SI unit. The Pascal (Pa) is defined to be one Newton (force) per square metre.

same same. Just that SI units don't suck because they are easily convertible from one to another without having to use/remember arbitrary conversion factors. And the mmHg and mmH2O units require that you use the force of gravity to convert them (9.807 m/s2) which is gay, and is not universal because the same real pressure would give you a different fluid column height on a different planet (or even a different height on different parts of Earth where the force of gravity is slightly different from elsewhere. It is a very small difference though!)

cheers

[89CAL]

Yeah your right, but in terms of absolute pressure, no?

Or is the fact that I havent done physics in 4 years really showing lol.......

[GTSBoy]

Makes no difference whether you're talking absolute or gauge pressure. It's all pressure. If you totally evacuate a volume you have zero absolute pressure in it. That's zero psi, zero kPa, etc etc. The gauge pressure (relative to 1 atm pressure outside the vessel) inside is -1 atm, -14.7 psi, -101.325kPa (assuming we're at sea level of course). Let a little bit of air in there, and you might have a pressure of 1 psi absolute, -13.7 psi gauge.

cheers

[89CAL]

Need to brush up on my physics lol............